A heavy-load drive apparatus used for an electronic watch in the past, as already proposed by the applicant in Japanese Unexamined Patent Publication (KOKAI) No. 62-274289, has a battery voltage detection circuit which, in order to drive a beeper device which is the heavy-load means, detects the voltage level of a battery which serves as the drive power supply and outputs a drive-limiting signal continuously when this voltage level is below a prescribed value, and a drive level control circuit which, when the above-noted drive-limiting signal is input, controls the drive of the beeper device so that it is driven with a drive current that is smaller than when being driven normally, thereby enabling the operation of an alarm function without sacrificing the timekeeping function when the battery is worn, and also enabling the detection of battery deterioration in the electronic watch. There is also disclosure of a method for stopping the drive to the heavy-load means when the voltage level is below a prescribed value.
However, in the above-noted prior art, in the case of using a battery which does not exhibit a rapid change in voltage, but rather deteriorates slowly, such as is the case with silver or lithium batteries, it is not possible to achieve the intended effect.
With interest in recent years in achieving environmental friendliness, watches have come into common use in which a compact power supply using a solar battery system formed by the combination of a solar cell and a storage device is used as the drive power supply instead of using the conventional battery.
In the drive source for a solar battery system, the storage device has a small capacity compared to that of a conventional battery and can undergo a large change in voltage over a short period of time, caused by a change in the charging environment, there also arising the problem of a large difference between the no-load voltage and the heavy-load voltage.
Therefore, when applying the above-noted solar battery, the following problems arise.
(1) In a prior art heavy-load drive apparatus which must limit the drive of the heavy-load means when the power supply voltage is detected as being blow a prescribed value, regardless of how low the power supply voltage level drops, the limited drive is started, and with a drive source such as a solar battery, the voltage drop causes the power supply voltage to drop extremely low, so that often operation of the timekeeping function cannot be assured. PA1 (2) In a heavy-load drive apparatus devised so that the heavy-load means is not driven when the power supply voltage level is below a prescribed value, in the case of a drive source exhibiting large voltage variations, such as a solar battery, the power supply voltage level drops below the prescribed value and often causes a stoppage of the drive to the heavy-load means, thereby causing the functional reliability to drop to an impractical level.
The above-noted problems will be described in more detail, with reference being made to FIG. 8.
FIG. 8 is a graph which shows an example of the time variations of the charging voltage in a solar battery.
In FIG. 8, the horizontal axis represents elapsed time, and the vertical axis represents the charging voltage of the solar battery, the various points indicated by squares being the measured values of power supply voltage level.
These measurements are performed at the time when performing normal drive of the heavy-load means, an so indicate the power supply voltage level when performing normal drive.
The solar battery illustrated in FIG. 8 exhibits a wide voltage variation from 1 V to 2 V. Consider the case in which the minimum voltage level Vmin at which timekeeping operation of the watch is guaranteed is 1 V, and a threshold value Vth is established at 1.15 V.
In the above-noted drawing, there is indicated a point P1 at which the power supply voltage level is 1 V.
When the power supply voltage level is below the threshold value, in the heavy-load drive apparatus which limits the drive of the heavy-load means, it continues to perform limited drive even at the point P1, so that ultimately when the power supply voltage level falls to below 1 V, it is no longer possible to assure proper operation of the watch.
In FIG. 8, although there is a plurality of points P2 which are below the threshold value but very close to 1.15 V, in a heavy-load drive apparatus devised so that the heavy-load means is not driven when the power supply voltage level is below the threshold value, at the above-noted points P2, the drive of the heavy-load means would stop.
The present invention was made to solve the above-noted problems in the prior art, and has as an object the provision of a heavy-load means for an electronic watch, which assures reliable operation of the watch and also performs proper drive in response to the power supply voltage level.